1. Field of the Invention
The present invention relates to an automatically guided vehicle used with an automatic guidance system in an automatic storehouse or the like.
2. Description of the Related Art
An automatic storehouse, for example, is furnished with an unmanned automatically guided vehicle for transporting articles. The guided vehicle travels along a track formed of, e.g., white lines or aluminum foil sheet on a floor. This track is composed of a basic track arranged like a circuit in the storehouse and sidewise or slantwise tracks diverging from the basic track. The guided vehicle runs along these tracks and reciprocates between the basic track and article transfer stations in the storehouse.
One such automatically guided vehicle is described in Jpn. Pat. Appln. KOKAI Publication No. 6-309029 (Prior Art 1). This guided vehicle is provided with two front wheels and two rear wheels. One of the two front wheels is a drive wheel that is rotated by means of a motor for travel, while the other is an idler wheel capable of free rotation. Likewise, the rear wheels include a drive wheel and an idler wheel. Each drive wheel is swingable around a vertical axis, and is driven so as to be pointed in a desired steering direction by means of a motor for steering. Each idler wheel is supported for rotation around a vertical axis. If the drive wheels are steered, therefore, the idler wheels change their directions as the guided vehicle travels.
On the other hand, an example of a three-wheel automatically guided vehicle is described in Jpn. Pat. Appln. KOKAI Publication No. 62-210173 (Prior Art 2). This guided vehicle is provided with one front wheel and two rear wheels. The front wheel is driven by means of a motor for travel, and one of the rear wheels is connected to the motor by means of a chain, sprockets, magnetic clutch, etc. When the vehicle is advanced, the clutch is released so that only the front wheel is driven. The front and rear wheels are connected by means of a steering cam system so that the rear wheels can be steered in association with the front wheel. In moving the guided vehicle of Prior Art 2 sideways, the respective steering angles of the front and rear wheels are both set at 90.degree.. If the clutch is thrown in, the front and rear wheels are rotated simultaneously. When the front wheel is rotated by means of the motor for travel, in this case, the rear wheels are rotated through the medium of the chain and the sprockets, whereupon the vehicle moves sideways.
The guided vehicle of Prior Art 1 has a four-wheel structure, including the two front wheels and the two rear wheels. If the floor or ground surface is rough, therefore, only three of the four wheels of the vehicle may possibly be allowed to touch the surface at a time, so that the vehicle may operate with one of its wheels off the surface, in some cases. In this situation, the vehicle is supported by only those wheels which are in contact with the ground surface, so that it is unbalanced and unstable as it travels, and sometimes meanders. It is not easy, moreover, to make this guided vehicle compact, owing to the use of the four wheels and the motors for travel and steering on both sides, front and rear. There is a tendency for modern automatically guided vehicles to require a depressed vehicle floor and more compact underside. The vehicle of Prior Art 1 cannot, however, meet this requirement. According to the automatically guided vehicle of Prior Art 2, on the other hand, the front and rear wheels are steered simultaneously by means of the cam mechanism as the vehicle moves sideways. Once the steering direction of any of the wheels shifts from any cause, therefore, it cannot be stabilized.